1. Field of the Invention
The present invention relates to an imaging apparatus.
2. Description of the Related Art
With recent advancements in the quality and speed of administrative paperwork at the office, for example, there is a growing demand for a color imaging apparatus such as a copier, a printer, or a facsimile machine with higher image quality and higher processing speed. A tandem color imaging apparatus is one type of imaging apparatus that is being developed in view of such a demand. For example, the tandem color imaging apparatus may include image forming units for the colors black (K), yellow (Y), magenta (M), and cyan (C), and may be configured to form a color image by transferring the different color images created by the image forming units in an overlapping manner onto a transfer material or an intermediate transfer medium that moves across the image forming units. It is noted that various types of the tandem color imaging apparatus have been proposed and developed into commercial products.
For example, FIG. 1 is a diagram illustrating an exemplary configuration of an electrophotographic direct transfer tandem color imaging apparatus as one type of a conventional tandem imaging apparatus.
In this example, latent images that are created on the surfaces of photoconductor drums 40Y, 40M, 40C, and 40K by a laser exposure unit (not shown) are developed by a developing unit (not shown) so that corresponding toner images (developed images) may be created. The photoconductor drums 40Y, 40M, 40C, and 40K having the toner images formed thereon are each rotated at a predetermined rotational speed by a gear decelerating mechanism (not shown) and a drive motor (not shown). The toner images are successively transferred and layered onto recording paper that is adhered to a conveying belt 210 by electrostatic force to be conveyed by the conveying belt 210 after which the toner of the transferred images is heated and pressurized by a fixing apparatus 213 so that a color image may be formed on the recording paper. The conveying belt 210 is arranged over a drive roller 211 and a driven roller 212 that are positioned parallel to each other with suitable tension. The drive roller 211 is rotated at a predetermined rotational speed by a drive motor (not shown), and in turn, the conveying belt 212 moves at a predetermined speed. The recording paper is fed to the conveying belt 212 at a predetermined timing by a paper feeding mechanism and is conveyed by the conveying belt 212 to move at the same speed as the conveying belt 212 so that an image may be formed thereon.
In the tandem color imaging apparatus as is described above, color drift may occur depending on the positioning of the images formed by the image forming units. Color drift may be caused by the relative deviations in the transfer positions of the different color images that are layered on top of each other at the recording paper. When such color drift occurs, a thin line image that is formed by layering plural color images may appear blurred, or white spots may be created around the periphery of a black character image when such black character image is set within a background image that is formed by layering plural color images, for example.
It is noted that color drift may be influenced by a constant component (DC component) that occurs on a constant basis and a variable component (AC component) that varies over the rotation period of a rotating element such as the photoconductor drum or the belt drive roller. The variable component occurring over the rotation period of the photoconductor drum may be primarily caused by transmission errors of a drive transmission system arranged at the photoconductor drum shaft (e.g., transmission errors caused by gear eccentricity and/or gear cumulative pitch deviation) or transmission errors of a coupling element that detachably couples the photoconductor drum to the drive transmission system (e.g., transmission errors caused by shaft tilting and/or shaft center deviation), for example.
In a tandem imaging apparatus, there may be variations in the amplitudes and phases of the positional deviation variable components (i.e., deviation from the desired transfer position) of the image forming units over a predetermined section of the transfer belt (i.e., transfer area covered by one rotation of each photoconductor drum), and such variations may lead to image quality degradation. Specifically, color drift variations may be reduced when relative positional deviations between different colors with respect to a given color are reduced. For example, with regard to color drift variable components, when the phases of the positional deviation variable components of a black image forming unit and a yellow image forming unit are the same, the positional deviation variable components may act to cancel out color drift variations between these colors. On the other hand, color drift variations are maximized when the phases of the positional deviation variable components differ by 180 degrees.
In the following, color drift variable components are described in greater detail with reference to FIGS. 2A and 2B. FIG. 2A is a diagram illustrating positional deviations of a photoconductor drum of the conventional tandem imaging apparatus. FIG. 2B is a diagram illustrating a positional deviation variable component of the conventional tandem imaging apparatus.
In FIG. 2A, even when the ON/OFF timing of light irradiated from a write unit 214 onto the surface of the photoconductor drum 40K according to an image pattern is constant, variations may occur in the rotational speed of the photoconductor drum 40K when there eccentricity in the rotating shaft of the photoconductor drum 40K so that variations may occur in the light irradiation to create crude density. Further, when the phase of the rotational speed variation differs for each photoconductor drum of each color, variations are created in the amount of positional deviations of the different colors to thereby result in color drift.
It is noted that eccentricity of the photoconductor drum 40K may be caused by a photoconductor drive gear (not shown) corresponding to a drive gear for the photoconductor drum 40K or a coupling element (not shown) for connecting the photoconductor drive gear and the photoconductor drum 40K.
With respect to the eccentricity component attributed to the photoconductor drive gear, since the photoconductor drive gears themselves are not exchangeable parts, measures may be taken to prevent positional deviations thereof upon manufacturing the tandem imaging apparatus by assembling the drive gears for the different color photoconductor drums in a manner such that their phases match. However, with respect to the eccentricity component attributed to the coupling element, since positional deviations of the coupling elements are caused by rotation of the coupling elements upon attaching/detaching the photoconductor drums, phase variations in the rotation of the photoconductor drums may inevitably be created. It is further noted the eccentricity of the photoconductor drum caused by detachment/attachment (maintenance) of the coupling member may have a greater influence on the rotation of the photoconductor drums compared to the eccentricity of the photoconductor drum caused by positional deviations of the photoconductor drive gear.
Thus, even when adjustments are made on the photoconductor drive gears to match the phases of the photoconductor drums 40K, 40C, 40M, and 40Y at the product manufacturing stage to minimize the occurrence of color drift, variations in the phases of the photoconductors drums may be easily created by exchanging the photoconductor drums thereafter so that color drift may not be effectively prevented.
Japanese Laid-Open Patent Publication No. 9-146329 discloses a technique for adjusting the rotational phase of photoconductor drums with respect to the color drift variable components occurring over the rotational period of the photoconductor drums. The disclosed technique involves adjusting the rotational phase of a photoconductor drum by forming color drift detection patterns on a transfer belt; detecting the patterns using CCD (charge coupled devices); extracting the maximum value, the minimum value, and the rise and fall zero cross points of a variation period (variation component) from the detected information; and averaging address values obtained from the four factors to detect a periodic rotational phase. In this way, influences of the rotation variations may be prevented from being reflected in the image being formed.
Also, Japanese Laid-Open Patent Publication No. 2003-145836 discloses a technique that involves forming an overlapping pattern of a combination of two colors, varying the rotational phases of corresponding photoconductor drums, and measuring the pattern width with a sensor. According to this technique, when the measured pattern width is a large value, this indicates that there are variations in the phase values of the photoconductor drums; on the other hand, when the measured pattern width is a small value, this indicates a match of the phase values. Thus, by repeating the process of varying the phases of the photoconductor drums and measuring the pattern width until the measured value of the pattern width becomes smaller than a threshold value, an optimal phase value may be detected.
However, according to the technique disclosed in Japanese Laid-Open Patent Publication No. 9-146329, the CCD is used as pattern detection means so that devices such as a timing generation circuit, a driver, and an amplifier circuit for amplifying the output signal of the CCD are required which leads to an increase in the price of the processing circuit.
Also, it is noted that the rotation variation value of the pattern formed on the transfer belt that is used as a reference for detecting the rotation variations of the photoconductor drums also represents rotation variations of other frequencies including rotation variations of a drive roller for driving the transfer belt and a roller supporting the transfer belt, for example. Therefore, in the case of detecting the phase and amplitude of a variation component based the zero cross points and peak values obtained from such a pattern, the resulting detection data may be significantly influenced by noise so that accuracy of the detection may not be ensured.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2003-145836, a phase value that can reduce the occurrence of color drift to a certain degree is detected. However, since the detection is performed by sequentially varying the photoconductor drum phase, the phase varying amount per sequence has to be reduced in order to improve the accuracy of the detection in which case the detection process may take a long time. On the other hand, when the phase varying amount per sequence is increased, although the detection time may be reduced, the detection accuracy may be degraded and color drift generation may not be adequately prevented.